On the local polaron model and its applications to intermediate valence systems

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Abstract
The local polaron problem in which a local electronic resonance in a wide conduction band is coupled to a local phonon mode is considered. Earlier work is extended by a more complete treatment of the case where a finite Fermi level is present which is important for intermediate valence materials. The authors demonstrate by variational and perturbational methods that there can be a renormalisation of the level width by a factor exp(-n), where n is the number of phonons in the polaron cloud. They discuss the conditions for this renormalisation to occur and in particular establish the importance of the closeness of the resonance to the Fermi level. The physical basis for the conditions and the relationship to the problem where only a single electron is present, where renormalisation only occurs if the level lies at the bottom of the band, are brought out. Renormalisation due to electron-hole pairs is also discussed. The authors suggest that in a lattice of resonances, renormalisation should lead to suppression of intersite scattering. The implications for intermediate valence systems are discussed.